In recent years, the use of advanced composite structures has experienced tremendous growth in the aerospace, automotive, and many other commercial industries. While composite materials offer significant improvements in performance, they require strict quality control procedures in the manufacturing processes. Specifically, non-destructive evaluation (“NDE”) methods are required to assess the structural integrity of composite structures, for example, to detect inclusions, delaminations and porosities. Conventional NDE methods, however, are very slow, labor-intensive, and costly. As a result, testing procedures adversely increase the manufacturing costs associated with composite structures.
Various systems and techniques have been proposed to assess the structural integrity of composite structures. One method to generate and detect ultrasound using lasers is disclosed in U.S. Pat. No. 5,608,166, issued Mar. 4, 1997, to Monchalin et al. (the “166 Patent”). The '166 patent discloses the use of a first modulated, pulsed laser beam for generating ultrasound on a work piece and second pulsed laser beam for detecting the ultrasound. Phase modulated light from the second laser beam is then demodulated to obtain a signal representative of the ultrasonic motion at the surface of the work piece. A disadvantage of such a system has been that in order to improve the systems ability to detect ultrasonic motion at the surface of the work piece a more powerful laser is required which may be impractical to construct or could damage the work piece due to excessive heating.
Another method to generate and detect ultrasound using lasers is disclosed in U.S. Patent Application Ser. No. 60/091,240 filed on Jun. 30, 1998 to T. E. Drake entitled “Method And Apparatus for Ultrasonic Laser Testing” hereafter DRAKE. DRAKE discloses the use of a first modulated, pulsed laser beam for generating ultrasound on a work piece and a second pulsed laser beam for detecting the ultrasound. Phase modulated light from the second laser beam is then demodulated to obtain a signal representative of the ultrasonic motion at the surface of the work piece. A disadvantage of such a system has been that in order to improve the systems ability to detect ultrasonic motion at the surface of the work piece a more powerful laser is required which suffers from the same problems as the '166 patent.
Another method to generate and detect ultrasound using lasers is disclosed in U.S. Pat. No. 5,137,361, issued Aug. 11, 1992, to Heon et. al. (the “361 Patent”). The '361 patent discloses the use of a laser to detect deformations of a oscillatory or transient nature on a remote target surface. The deformations on the remote target surface can be produced by an ultrasound wave or other excitation. Light from the laser is scattered by the deformations, some of which light is collected by collecting optics and transmitted via a fiber optic to a beam splitter which deflects a small portion of the collected light to a reference detector and delivers the remaining portion of the light to a confocal Fabry-Perot interferometer, which generates an output signal indicative of the deformations on the remote target surface. The reference detector measures the intensity of the scattered laser light at the input of the interferometer to generate a reference signal. A stabilization detector measures the intensity of the scattered laser light at the output of the interferometer to generate a prestabilization signal. The ratio of the reference signal to the prestabilization signal is used to generate a final stabilization signal which drives a piezoelectric pusher inside the interferometer to adjust its resonant frequency. A disadvantage of such a system has been that a portion of the signal is lost at the beam splitter when sent to the reference detector. Again in order to improve the systems ability to detect ultrasonic motion at the surface of the work piece a more powerful laser is required.
An alternate to using a more powerful laser is to decrease the working distance to the part and/or increase the collection aperture size. This reduces the F-number of the optical system and has the disadvantage of a corresponding reduction in the working depth of field (DOF). DOF is a measure of how far away from the ideal focal plane an object can be and still maintain acceptable performance. Lower F-number designs generally result in smaller scan area capability and often require active focusing lens assemblies to maintain efficient light collection while scanning complex shaped components. Large collection apertures require the use of single-mirror optical scanning systems, usually in a two-axis gimbal configuration, that are cumbersome and generally slow.
Moreover, there is a need for a ultrasonic laser system which improves detection capabilities of the system to detect ultrasonic motion at the surface of the workpiece using practical lasers without damaging the workpiece and functioning with sufficiently large DOF.